Security researchers demonstrated Thursday flaws that can allow hackers to take over mobile point-of-sale (mPOS) devices from different manufacturers by inserting rogue cards into them.

Despite a patch being available since April, some devices remain vulnerable.

Jon Butler, the head of research at MWR InfoSecurity and one of his colleagues who prefers to be known only as Nils, have investigated six of the most popular mPOS devices available on the market that support the EMV (Chip-and-PIN) standard, they said at the Black Hat security conference in Las Vegas.

These devices have a small screen, a smart card reader and a PIN input pad. They run a Linux-based OS and communicate via Bluetooth with mobile payment apps installed on smart phones.

The MWR researchers found that despite looking different on the outside, 75 percent of the devices they tested were based on the same underlying platform.

In some devices they found vulnerabilities in the firmware update mechanism that allowed them to execute commands as root. They also found a stack-based buffer overflow vulnerability in the certified EMV parsing library that allowed them to take complete control over all devices using a specially programmed smart card.

The manufacturers of the devices were not named because some are still likely vulnerable.

To demonstrate that they can gain complete control over the screen and input pad of such a device, the researchers used a rogue card to install and run a game similar to Flappy Bird on one of them.

In a practical attack scenario, a fraudster could go into a store that uses such devices, claim to buy something, input his rogue card into a device and compromise it with code that would capture the card details and PINs of customers who later use it, the researchers said. The attacker or an associate could later return with a different card to extract the information.

Attackers would not be able to clone the chip of EMV cards, but it would be possible to clone the rest of the card information and use the resulting counterfeit card together with the captured PIN in a country that doesn't use the EMV standard.

The vulnerability was reported to the platform vendor, who was surprisingly cooperative and released a patched version of the EMV library in April, the researchers said. The process was not straight-forward because the new library had to go through the EMV certification process again, which highlights the fact that such standards do not allow for timely security updates, they said.

Despite most of the affected devices having remote firmware update capabilities, some vendors have not yet released updates containing the patched EMV library. Nils said that he is aware of at least one vendor who hasn't pushed updates to its customers yet. He doesn't know the status of all devices, because for some of them they didn't create merchant accounts in the first place that would give them access to updates.

The researchers have not yet fully investigated all attack vectors, but they believe it could also be possible to compromise mPOS devices from a smart phone infected with malware. In at least one case they found issues with a vendor's mobile application that suggest such an attack is possible.

It might also be possible to attack the smart phone from a compromised mPOS device and then upload the captured data over the phone's Internet connection. However, testing this could affect the vendor's back-end systems, so because of legal reasons the researchers didn't look further into it.

Despite the issues found, Butler thinks that mobile POS devices like the ones his team tested have the potential to be more secure than traditional POS devices. They're simple devices and there's not much that can go wrong if the implementation is done right, following security best practices.

One of the advantages they have is that they are theoretically easy to update. The vendor can push an update through the mobile app which then pushes it to the paired mPOS device over Bluetooth. The updates are digitally signed, so they cannot be tampered with.

However, vendors should stop viewing chip-enabled cards as part of a trusted system, Butler said. It's not like every card that can be inserted into one of these devices has been freshly issued by a bank, he said.

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